Hollow, air cooled blades and vanes are commonly used in modern gas turbine engines. These components have an internal cavity through which air flows during engine operation. This air is discharged through holes, called cooling holes, which are present in the airfoil section and sometimes present in the platform and tip. See, e.g., commonly assigned U.S. Pat. No. 4,474,532 to Pazder. The passage of air through and over the blade or vane extracts heat from the component surface, allowing use of the component even when the gas stream temperature exceeds the melting temperature of the alloy from which it is made.
Some gas turbine engines are designed so that during engine operation, the tip portion of the rotating blades rubs a stationary seal, and limits the leakage of working medium gases in the axial flow direction. While the seals are usually more abradable than are the blade tips (so that during such rub interactions, a groove is cut into the seal), the blade tips do wear, and the blades become shorter. As the blades accummulate service time, the total tip wear increases to the point that eventually, the efficiency of the blade and seal system is reduced, and the seal and blades need to be repaired or replaced.
The tips of worn blades can be repaired, and the length of the blade increased, by the addition of weld filler metal to the tip using any of the welding techniques (typically arc welding techniques) known to those skilled in the art. During such a weld repair operation, cooling holes near the blade tip are susceptible to being welded shut. These cooling holes must then be redrilled, e.g., using conventional laser or electrodischarge machining (EDM) techniques, before the blade can be used again.
However, with some blades used in advanced gas turbine engines, it is not practical to redrill the cooling holes after weld repairing the tip. This is due to the complex geometry of the holes, sometimes referred to as diffusion or shaped holes. See, e.g., U.S. Pat. Nos. 3,527,543 to Howald and 4,197,443 to Sidenstick. Air discharged through these holes forms an insulative film over the surface of the blade during engine operation, which further protects the blade from the effects of operating at very high temperatures. Shaped holes have a nonuniform cross section; for example, the entrance or metering portion of the hole generally has a very small diameter (in the range of about 0.010-0.050 cm (0.005-0.020 in.)) while the exit or diffuser portion of the hole has a relatively large diameter (in the range of about 0.090-0.115 cm (0.035-0.045 in.)). Furthermore, shaped holes may have a square cross section at the metering portion and rectangular cross section at the diffuser portion.
As can therefore be appreciated, the formation of shaped holes can be a difficult and technically complex operation. Consequently, if a blade having shaped holes is weld repaired, such a repair operation is preferably done so that the holes are not welded shut and do not have to be redrilled. Accordingly, what is needed is a method for weld repairing components having shaped cooling holes so that the holes are shielded from the molten filler metal and do not need to be redrilled after the weld operation.
U.S. Pat. No. 3,576,065 to Frazier discloses one method for weld repairing hollow gas turbine engine vanes having cooling holes with a constant diameter of about 0.125 cm (0.050 in.). Prior to welding, cylindrical ceramic inserts are inserted into and plug each of the holes; it is stated that the inserts prevent weld filler metal from entering the holes. Cylindrical inserts would not fill and therefore not protect shaped holes from molten filler metal, due to the noncylindrical and nonuniform cross section of the holes. Furthermore, the small diameter of shaped holes would require equally small diameter ceramic inserts. Such inserts, even if fabricable, would be extremely brittle, difficult to handle, and therefore have questionable utility.